Find common elements in a file - perl

Question

The program that I would like to write has the same aim of the File row confrontation. This time the file I have is put in a different way:
1 2
1 3
1 4
2 1
2 3
2 4
2 5
3 1
...
8 6
8 7
8 9
9 8
I want to find:
when the first element of a row appears in the second position of the other rows and if the first element of the subsequent rows appear alongside the row taken in exam;
if it found then I want to print "I have found the link x y";
if the "link" exists, then I want to count how many "neighbours" they share, where by eighbours I mean how many elements in the second column they have in common and print "I found z triangles".
The file is sorted.
In this case the program will start founding the first "couple" 1 2 in the file but reversed and it will find it at the 4th row (2 1). Then it looks if the 3 ( second row and neighbour of 1) is also present in 2 ( and it is the case because it exists 2 3) and so on. At the end it will found that the "there is the link 1 2" and it "found 2 triangles" (1 - 2 - 3 and 1 - 2 - 4). I think the answer sould not be so different from the answer in the upper link, but I don't know how to arrange the files from a file made like this.

Answer 1

The first part of the problem is to find only the index of the inverted matching pairs? While reading this problem yesterday I had the feeling that grep may be of use;
#!usr/bin/perl
use warnings;
use strict;
my #parry;
while (<DATA>){
push #parry, [split(' ',$_)];
}
##remind is reverse matched indices;
my #remind = grep {
my $ind = $_;
grep { #reverse #{$parry[$_]} == #{parry[$ind]} did not appear to work.
#{$parry[$_]}[0] == #{$parry[$ind]}[1] &&
#{$parry[$_]}[1] == #{$parry[$ind]}[0];
} 0..$#parry
} 0..$#parry;
grep { print $_,': ',#{$parry[$_]},$/ } #remind;
__END__
1 2
1 3
1 4
2 1
2 3
2 4
2 5
3 1
8 6
8 7
8 9
9 8
output is
0: 12
1: 13
3: 21
7: 31
10: 89
11: 98
from here you then want to find say for
7[0] 7[1] (3 1) with neighbour row 6 and 8 with col 2?
6[1]
7[1] (1 5) and/or
7[1] (1 6) exist in the original set (in #parry)?
8[1]
Which they do not so no triangle.